Bragg-Gray theory and ion chamber dosimetry for photon beams.

The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively in this paper. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, Fair, of the absorbed dose resulting from photon interactions in an air cavity, DPA(air), to that in air under the condition of charged-particle equilibrium, DCPE(air), has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, Fair, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, Fair, is 0.29 for a 150 kVp beam and 0.27 for a 240 kVp beam compared to 0.006 for a 60 Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.